1. Field of the Invention
This invention relates to a method of electrolytically machining a workpiece, in particular a workpiece made of a hard metal using a pulsed electric current based on the principle of electrolysis, and also an apparatus for attaining the method.
2. Description of the prior art
The electrical discharge machining process is used mainly for molding molds having irregular shape. The surface of a workpiece to be subjected to electrical discharge machining is a congregation of minute craters whose surface roughness is about 5 .mu.m R max or more, in general, under normal finished condition. Further, since the surfaces of workpieces to be subjected to electrical discharge machining have a large number of minute cracks, if mold parts having such surfaces are put to use, the cracks will grow due to loading and thermal shock exerted thereto, resulting in failure or damage thereof. Therefore, the mold parts subjected to electrical discharge machining require, in most cases, improvement of surface roughness thereof and removal of layers which changed in properties due to electrical discharge machining and which contain a large number of minute cracks.
For removing such layers whose properties were changed due to electrical discharge machining, lapping and electrolytic finishing have heretofore been applied.
Because of irregular or complicated shapes of mold parts to be finished, the lapping process has heretofore been conducted manually in most cases, and disadvantageous in that this operation must be done by skilled workers for a long time. Whilst, electrolytic finishing process is very effective as compared with the lapping process, since it can be carried out by subjecting the surface of a workpiece whose shape has been created by electrical discharge machining to electrolytic machining to some degree; that is, eluting the surface of the workpiece electrolytically so that the surface layers whose properties were changed due to the electrical discharge machining can be removed, and at the same time the surface roughness thereof can be improved. In the case of electrolytic finishing work using single polarity pulses, however, it is possible to process or work most of metals including iron by using a workpiece as anode and a tooling electrode as cathode and applying an electric current across them in a neutral electrolyte (which is normally an aqueous solution containing sodium nitrate). However, in case hard alloys containing a great deal of WC or TiC particles are subjected to electrolytic finishing, because Co which is a binder elutes into the aqueous solution, but WC or TiC forms a film of WO.sub.3 or TiO.sub.2 on the surface of the workpiece, it is impossible to proceed working of them. For this reason, electrolytic finishing alloys is carried out effectively in an aqueous solution containing NaOH which has a property to elute WO.sub.3 or TiO.sub.2. However, the aqueous solution containing NaOH shows an intense alkalinity and is harmful to the workpiece, and therefore it is rather unsuitable for practical application.
To eliminate such a disadvantage, it has been proposed to conduct electrolytic finishing of a hard alloy in an aqueous solution containing sodium nitrate using alternating current. According to this method, the polarities of the workpiece and the tooling electrode are reversed alternately so that Na.sup.+ ions gather on the surface of the cathode and react with water to form a layer of an aqueous solution containing NaOH to thereby enable electrolytic finishing of the hard alloy to be proceeded, so that the surface of the workpiece can be finished considerably satisfactorily. However, this method is disadvantageous in that the tooling electrode as well as the workpiece are consumed due to the electrolytic elution, and in addition to this, reduction in the time for finishing work and further improvement in the surface of workpieces to be finished thereby are required.